Ultrasonic pickling



Nov. 27, 1962 H. F. osTERMAN, JR., ETAL 3,066,084

uLTRAsoNIc PICKLING Filed Aug. 10. 1959 2 Sheets-Sheet 1 INVENTORS Harry F. OstermomJr. R sse L.Uphoff ATTORNE -v 01 NN O NN NN Nov. 27, 1962 Filed Aug. 10, 1959 H. F. osTERMAN, JR., ETAL 3,066,084

ULTRASONIC PICKLING 2 Sheets-Sheet 2 No'lnduced Ultrosonlc Energy Fi 9.3 I

.E o x I i l l l l l l l l l J O l 2 3 4 5 7 e 9 lo PoIentiaI UIIrusonic Energy Induced Z FIQA E During EnIIre Pickling Cycle I v Y l l l I l I l l I l I 0 l 2 3 4 5 6 7 8 9 IO Potential Ultrasonic Energy Induce'd Z F|Q5 E During ParI Of Picklinq Cycle P o X\ I) r x l 1 l l l O l 2 3 4 5 6 7 8 9 IO Poeniial Fig.6

`F`UIIras-onic Energy //f I l I I 50 95 |00V PICkII'Iq Time- PerCen INVENTORS Harry F.0stermun,Jr.

Russel L. Uphoff ATTORNEY United StatesPatent O 3,066,084 ULTRASONIC PICKLING Harry F. Osterman, Jr., Stamford, Conn., and Russel L.

Upholf, Murrysville, Pa., assignors to Jones & Laughlin Steel'Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 10, 1959, Ser. No. 832,744 3 Claims. (Cl. 204-144) This invention relates to a method for pickling metallic articles to remove oxide scale and surface imperfections, and more particularly to a pickling method employing ultrasonic energy to shorten the required pickling time.

In the conventional manufacture of flat steel products, an ingot is first rolled into a slab which is subsequently hot rolled into an elongated strip called hot band. After hot rolling and annealing, the hot band is covered with a scale of oxides which is removed by passing the strip through a pickling line including one or more sulfuric acid baths which remove the oxide scale by dissolving the surface layer of the metal. During the pickling operation, an undesirable black coating of smut is formed on the surface of the steel; and in certain operations, where the finished product must be plated or lacquered, the smut must be removed in an additional process.

In the usual pickling operation continuous hot band strip is passed through one or more elongated sulfuric acid tanks each of which may, for example, measure about 60 to 80 feet in length. In order to complete the pickling operation, the hot band must remain immersed in the sulfuric acid tank or tanks for a predetermined period of time as it passes therethrough. For plain carbon steel, for example, the required pickling time is between about two and three minutes in a 12% sulfuric acid bath having a temperature of about 170 F. As will be understood, the speed of the strip through a pickling tank of any given length is a function of the amount of time required to complete the pickling cycle. That is, as the speed of the strip is decreased, the permissible length of the tank is also decreased. On the other hand, the speed of the strip and the production rate may be increased by increasing the length of the pickling tank or tanks; however, this requires a relatively large capital investment.

It is well known that ultrasonic energy may be used to increase the pickling rate of a sulfuric acid bath of the type described above. Such an arrangement has been proposed in the past; however, it contemplates the use of a plurality of ultrasonic transducers spaced along the entire path of travel of the strip in the pickling bath due to the fact that each transducer will activate only a relatively small portion of the bath. By using such transducers, the length of the pickling tanks would be reduced for any given strip speed; however, the savings realized by decreasing the length of the pickling tanks could be more than offset by the added cost of the transducers.

It is a primary object of this invention to provide a method for rapidly descaling metal by reacting it with a bath which dissolves the metal.

Another object of the invention is to provide a method which will increase the production rate of a pickling line by use of ultrasonic energy at a relatively low cost. A1- though the illustration given above involves pickling steel in sulfuric acid, the invention is by no means limited thereto and may be used in any pickling operation for any material.

Another object of the invention resides in the provision of a low cost ultrasonic pickling method wherein the smut formed in the usual steel pickling operation is eliminated from the surface of the strip.

As will become apparent from the following detailed description, the invention resides in the discovery that ultrasonic energy materially decreases the required pick- 3,066,084 Patented Nov. 27, 1962 ICC ling time during only a portion of the pickling cycle. That is, when a metallic article such as steel is immersed in sulfuric acid in the presence of ultrasonic energy, the pickling cycle is initially accelerated over that obtained in conventional pickling practice where no ultrasonic energy is employed. Beyond this initial period, however, the ultrasonic energy has a reduced effect on the pickling process. Accordingly, the present invention provides a method wherein ultrasonic energy is applied only during the initial part of the pickling cycle when the sound most accelerates the pickling operation. In a continuous pickling line, for example, this enables the use of a fewer number of transducers spaced along the length of the strip in the pickling bath while obtaining substantially the same results achieved when ultrasonic energy is employed for the entire pickling cycle.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:

kFIGURE 1 is a schematic illustration of the pickling process of the present invention, showing the location of ultrasonic transducers within the pickling tanks;

FIG. 2 is a top view of the apparatus shown in FIG. l;

vFIG. 3 is a plot of potential versus time for a conventional pickling process wherein ultrasonic energy is not employed to accelerate the pickling cycle, said potential being generated between a scaled steel article and an electrode of clean steel immersed in a pickling bath;

FIG. 4 is a plot of potential Versus time for the case where ultrasonic energy is applied during the entire picklin g cycle;

FIG. 5 is a plot of potential versus time for the process of the present invention wherein ultrasonic energy is applied only during a portion of the pickling cycle; and

FIG. 6 is a plot ofthe percentage of pickling time versus potential for one embodiment of the process of the present invention.

Referring to FIGS. l and 2, hot band strip 10 containing an oxide scale is passed through two sulfuric acid pickling tanks 12 and 14.` Although two tanks are shown herein, it is to be understood that the number of tanks depends upon their length, the speed of the strip 10 and the required pickling time. The tanks 12 and 14 have rolls 16 and 18 of noncorrosive material which hold the strip under the surface of the sulfuric acid in the tanks; whereas, pinch rolls 2t) and 22 guide the strip into and out of the tanks.

As the strip 1li passes through tanks 12 and 14, the sulfuric acid chemically reacts with the strip to remove oxide scale. It has been found that at least a portion of the oxide removal in the acid bath is due to electrochemical reaction. In this process the steel base is anodic with respect to the adhering oxide formation. Thus, if an electrode of clean steel is placed in close proximity to the strip being pickled, a varying electrical potential will exist between the strip and the electrode; and this potential will remain constant when all of the scale has been removed.

Referring now to FIG. 3, it can be seen that when no ultrasonic vibrations are induced in the sulfuric acid bath, the potential that is generated between the strip and a clean steel electrode immersed in the bath initially increases very rapidly and then decreases, again very rapidly, until point X is reached. Between points X and Y the potential again increases over a relatively long period of time. At point Y the potential again decreases until the end point Z is reached where the potential remains constant regardless of the amount of time that the strip remains within the sulfuric acid. As can be seen from FIG. 3, the end point does not occur at zero potential, apparently indicating that pickling is still in progress. Due

to the fact, however, that there are several diierent metals in the sulfuric acid bath, an electrolytic cell is formed. Since at least one such cell cannot be isolated from the measuring circuit, an additional potential is always present even though the pickling cycle is completed.

If a plurality of ultrasonic transducers were spaced along the entire length of the strip 10 during the pickling operation, the strip would be under the inuence of ultrasonic energy during the entire pickling process. In FIG. 4, it can be seen that under these conditions the potential again initially increases and then decreases until point X is reached as was the case in FIG. 3. The period of time required to travel between points X and Y, however, is materially reduced. That is, the slope of the curve between points X and Y in FIG. 4 is much less than the corresponding slope between points X and Y in FIG. 3. It will be noted, however, that the slopes of the curves between point Y and the end point Z in both FIGS. 3 and 4 are essentially the same, meaning that the piekling process is not materially accelerated by the use of ultrasonic energy during the portion of the cycle between point Y and the end point Z.

In FIG. 5, the pickling cycle of one embodiment ofthe present invention is shown wherein ultrasonic energy is applied only between points X and Y where the slope of the curve can be reduced by the application of ultrasonic energy. Thus, the ultrasonic vibrations are induced only during that part of the cycle, and conventional pickling takes place during the remainder of the cycle with the result that the period of the pickling cycle is only about two-thirds that of conventional practice shown in FIG. 3. The application of ultrasonic energy is delayed in the process until about tive percent of the pickling cycle has elapsed. This is desirable since it takes this amount of time for the hot band to reach the solution temperature where maximum attack rate occurs. When point Y has been reached about one-half of the cycle time has elapsed. Although the conditions outlined above are most desirable, it can be said in general that application of ultrasonic vibrations during about the rst fifty percent of the cycle will materially increase pickling eiciency.

In FIG. 6 the ideal pickling cycle is shown. This cycle is the same as that of FIG. 5 except that ultrasonic vibrations are again induced in the bath during the last live percent of the cycle time. It will be noted that just before point Z is reached, the rate of change of potential decreases until the end point or steady state potential condition is reached. Accordingly, by applying ultrasonic energy during the last five percent of the cycle, the eciency of the process is increased further by promoting rapid reaction conclusion and insuring the removal of all pickling residue. In addition to the foregoing advantages, pickling in this manner eliminates or materially reduces the formation of smut on the surface of the strip which would otherwise have to be removed if the iinal product is lacquered, plated or otherwise linished in a manner requiring a clean surface.

Referring again to FIGS. 1 and 2, the apparatus for etecting the pickling cycle shown in FIGS. 3-6 in a continuous pickling line includes a plurality of ultrasonic transducers 24 spaced between the first ve percent and fty percent of the total pickling line length. A second plurality of ultrasonic transducers 26 is spaced along the last tive percent of the total length, while an ultrasonic generator 28 is connected to the transducers 24 and 26 through leads 30. Thus, as the strip 10 travels through tanks 12 and 14, it will pass through the cycle graphically illustrated in FIG. 6. Of course, if batch pickling is employed where the steel remains stationary, one or more transducers in the bath could be cycled to operate only between five percent and fty percent of the pickling cycle as well as the last five percent of the cycle.

The invention thus provides a means for employing ultrasonic energy to reduce the required pickling time with the use of ultrasonic energy while at the same time minimizing the number of ultrasonic transducers required to achieve a given production rate. Although the invention has been shown in connection with a certain specic ernbodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In the process of descaling metal by reacting it with a bath which dissolves the scale on the metal and in which an electrical potential exists between the scaled metal and a scale-free piece of the same kind of metal immersed in the bath, the improvement comprising inducing ultrasonic vibrations in the bath only during the portion of the descaling process when that electrical potential increases, whereby that increase is accelerated and the total time required for descaling is shortened.

2. The process of claim l in which the time during which ultrasonic vibrations are induced is about one-half the total time required for descaling.

3. In the process of descaling metal by reacting it with a bath which dissolves the scale on the metal and in which an electrical potential exists between the scaled metal and a scale-free piece of the same kind of metal immersed in the batn, the improvement comprising inducing ultrasonic vibrations in the bath during about the rst one-half of the total time required for descaling while said electrical potential increases, and inducing ultrasonic vibrations in the bath for a second portion of the descaling process when the electrical potential remains reasonably constant because descaling is completed, the second time being about ve percent of the total time required for descaling.

References Cited in the le of this patent UNITED STATES PATENTS 1,965,399 Wehe July 3, 1934 2,165,326 Yerger et al July ll, 1939 2,522,082 Arnold Sept. 12, 1950 2,744,860 Rines May 8, 1956 2,845,077 Branson July 29, 1958 2,888,939 Nitsche June 2, 1959 2,893,707 Gulton July 7, 1959 2,894,860 Englehardt July 14, 1959 FOREIGN PATENTS 748,485 Great Britain May 2, 1956 

1. IN THE PROCESS OF DESCALING METAL BY REACTING IT WITH A BATH WHICH DISSOLVES THE SCALE ON THE METAL AND IN WHICH AN ELECTRICAL POTENTIAL EXISTS BETWEEN THE SCALED METAL AND A SCALE-FREE PIECE OF THE SAME KIND OF METAL IMMERSED IN THE BATH, THE IMPROVEMENT COMPRISING INDUCING ULTRASONIC VIBRATIONS IN THE BATH ONLY DURING THE PORTION OF THE DESCALING PROCESS WHEN THAT ELECTRICAL POTENTIAL INCREASES, WHEREBY THAT INCREASE IS ACCELERATED AND THE TOTAL TIME REQUIRED FOR DESCALING IS SHORTENED. 